To counter matt_the_bass's example, I live in the NE US, I have always used the method (in this case, window upstairs open, window downstairs open, natural draf

To counter matt_the_bass's example, I live in the NE US, I have always used the method (in this case, window upstairs open, window downstairs open, natural draft cooling the house by dragging the air through), but there is absolutely no level of filtering that will stop all pollutants and allergens from entering the house as long as you are not filtering 100% of the air. Last time I noticed, nobody makes a window-sized AC-esque filter that works efficiently down to PM2.5 levels... and doing so would also eliminate said window being a window.